Thermoelectric based generators have been used successfully and reliably for the past 40 years to power deep space probes. These solid-state devices rely only on a temperature gradient to produce electricity, and are thus an attractive way of reducing our demand on fossil fuels. A relatively new application of thermoelectrics is in the area of automobile waste heat recovery. In a typical car, only 25% of the gasoline that is combusted is actually used to move and power the car, while the rest is lost as heat. If some of that heat were recaptured and stored, it would increase the fuel economy of automobiles, and reduce the overall demand for fossil fuels.
Currently, thermoelectric devices have been used only in niche applications such as space craft power generation. In order for them to be attractive for such a large-scale application, the thermal-to-electric conversion efficiency must be increased. Additionally, due to the significant number of automobiles, materials cost, materials abundance, and toxicity also become concerns with regard to the utilization of these devices.
Magnesium silicide and related alloys are attractive candidates because they are composed of abundant and low-cost elements and are relatively non-toxic when compared to their state-of-the-art counterparts, PbTe and CoSb3, which operate in the same mid-temperature range (400 K to 800 K). The family of magnesium W compounds, including Mg2Si and Mg2Sn and their alloys, crystallize in the antifluorite structure with Si in face centered cubic positions and Mg in tetrahedral sites. They have been studied as potential thermoelectric materials in the mid- to high-temperature range for the past sixty years. It is theorized that these compounds can achieve high zT values due to their large effective masses, high mobilities, and relatively low lattice thermal conductivities.
Mg2Si-based compounds are typically synthesized on a small scale via melt synthesis or casting. Scaling up these methods is problematic due to the high vapor pressure and reactivity of magnesium, which can lead to the loss of magnesium and poor control over stoichiometry. Off-stoichiometric material can lead to vacancies and other defects in the crystal structure which affect the extrinsic carrier concentration and carrier mobility. A similar problem was found in the La3-xTe4 system, where high temperature synthesis leads to poor stoichiometric control that can be solved by switching to a mechanochemical synthetic method. Mechanical alloying or high-energy ball-milling can be an attractive technique for producing large-scale quantities of materials that usually require complex synthetic processes at elevated temperatures.
Ball-milling is often performed with materials that are very brittle, and thus respond well to the fracturing and welding that occur in milling. For materials that have a higher malleability, such as magnesium, ball-milling often leaves incomplete product formation, and aggregation of the malleable material, even with extended milling times. Earlier work with this technique generally resulted in incomplete product formation, leaving a significant amount of unreacted elements. Originally, Schiltz and co-workers theorized that the phase pure compound could be made after 1400 hours of continuous operation using an impact mill (M. Riffel and J. Schiltz, Scripta Metallurgica et Materiala, 1995, 32, 1951-1956). In an approach to control the kinetics of ball milling, Li and Kong used a lower impact energy, friction-driven, planetary mill; however, even after 100 hours of milling, they could not synthesize phase pure Mg2Si (G. H. Li and Q. P. Kong, Scripta Metallurgica et Materialia, 1995, 32, 1435-1440). Niu and co-workers sought to optimize the Li and Kong synthesis by increasing the RPMs of the planetary ball mill, ball-to-powder ratio, and time, yet still could not produce phase pure product after 30 hours of milling (X. Niu and L. Lu, Advanced Performance Materials, 1997, 4, 275-283).
In addition to failing to produce a phase pure product, these groups all reported significant contamination from the milling media during the ball-milling process. In other attempts to obtain a phase pure product, various groups ball-milled the starting materials, then subjected the finely divided powders to thermal treatment either by annealing or sintering via hot-pressing or spark plasma sintering to make phase pure Mg2Si bulk compacts. However, the problem with the high vapor pressure of Mg remains, and this can lead to catastrophic failure of the annealing vessel or the graphite dies. What is needed is a method of making a substantially phase pure compound comprising an alkali or alkali earth metal and a Group IV element. Surprisingly, the present invention meets this and other needs.